Nanometric films of iron oxides (Fe3O4, α and γ Fe2O3) of high crystalline order and purity are epitaxially grown on α-A12O3(0001) by atomic oxygen assisted MBE. A complete characterization of the films structure has been performed by in situ LEED and RHEED, and ex situ GIXRD using synchrotron radiation. The films grown at room temperature and post annealed at 400°C and 700°C (po2=10−6 Torr) are respectively metastable γ-Fe2O3 (111) and α-Fe2O3 (0001). For a substrate temperature of 450°C during growth, Fe3O4 (111) is directly obtained. GIXRD shows an in-plane expansion of the films, which decreases with thickness (0.8 and 0.2% for film thickness of 20 and 80 Å, respectively).